1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier and a laser-beam printer, and in particular to an image forming apparatus including a fixing section including an endless belt, a heater that contacts the inner surface of the endless belt, and a nip portion forming member that forms a fixing nip portion together with the heater via the endless belt.
2. Description of the Related Art
An image forming apparatus for use in an area where the commercial power source voltage is around 100 V (for example, 100 V to 127 V) may be used in an area where the commercial power source voltage is around 200 V (for example, 200 V to 240 V). In such a case, the maximum power that can be supplied to the heater of the fixing section may be quadrupled compared to a case where the apparatus is used in the 100-V area. As the maximum power that can be supplied to the heater becomes larger, a harmonic current, flicker, or the like produced in heater power control such as phase control and wave number control becomes more significant. In addition, power produced when thermal runaway occurs in the fixing section is quadrupled, which requires a more responsive circuit. Therefore, in many cases, a single apparatus that may be used in both the 100-V area and the 200-V area is provided with heaters with different resistance values for the respective areas.
Meanwhile, a technique for changing the resistance value of a heater using switching units such as relays is proposed to provide a universal apparatus that may be used in both an area where the commercial power source voltage is 100 V and an area where the commercial power source voltage is 200 V. Japanese Patent Laid-Open No. 7-199702 proposes an apparatus including first and second resistance heating bodies provided on a heater substrate. Switching can be made between a first operating state, in which the first and second resistance heating bodies are connected in series with each other, and a second operating state, in which the first and second resistance heating bodies are connected in parallel with each other, to change the resistance value of the heater in accordance with the commercial power source voltage in order to allow the apparatus to be used in both the 100-V area and the 200-V area.
According to the technique for switching the first and second resistance heating bodies between the series connection state and the parallel connection state in accordance with the commercial power source voltage, the resistance value of the heater may be changed without changing the heating region of the heater. In other words, both the two resistance heating bodies generate heat irrespective of whether the apparatus is used in the 100-V area or the 200-V area. Thus, the temperature distribution in the fixing nip portion in the recording material conveying direction is the same irrespective of the area of use. This is advantageous in that the fixing performance of toner images is not affected by the area where the apparatus is used.
A technique that uses power shutoff elements such as relays is widely employed as safety measures for a case where power supply to a heater may not be controlled to cause thermal runaway of the heater. In addition, a technique in which relays are provided on both sides of a heater to insulate the heater from an alternating-current power source for electric shock prevention is also known. However, separately providing power shutoff relays as safety measures to the apparatus in which the first and second resistance heating bodies are switched between the series connection state and the parallel connection state using the connection state switching relays as described in Japanese Patent Laid-Open No. 7-199702 would increase the cost.